1. Field of the Invention
The invention relates to centering and cooling equipment for a hydraulic vibration generator with a pulsation generator pressurising a cylinder containing a movable piston, heated pressure fluid being extracted from each cylinder chamber and cool pressure fluid fed to it.
2. Description of the Prior Art
Such a hydraulic vibration generator is mainly used for driving vibratory compactors, but also for ramming and pulling machines used in construction work, for vibratory sieves, conveyors and stone breaking tools.
With these the piston can reciprocate in the cylinder or as an alternative the cylinder can reciprocate on the piston. These two methods are used for linear reciprocating motors.
Where the vibration generator is in the form of an angularly-oscillatory motor the oscillatable piston is rotated to and fro in a cylinder about an axis running in the lengthwise direction.
German OS No. 2 231 106 covers a vibration generator in which the phase difference between two pressure sources can be varied, thus allowing stepless adjustment of the cylinder/piston stroke and hence of the output.
With such a vibration generator and particularly at high vibration speeds and outputs, friction between the piston and the cylinder and also continuous contraction and expansion of the fluid under pressure generates a considerable amount of heat which is not dissipated via the pressure fluid itself since there is only a pulsating movement, i.e. a reciprocating movement, of the pressure fluid between the piston and the pressure source. However, heating of the fluid under pressure has an adverse effect on the lubrication characteristics between the sliding surfaces so that due to leakages fluid under pressure escapes and divergence of the piston results. Apart from this damage to the seals can result.
Control of the temperature of the pressure fluid in the cylinder can only be achieved by additional constructional features, for example by means of a heat exchanger round the cylinder, since in most cases sufficient heat dissipation by convection with the surrounding medium and radiation to adjacent components at lower temperatures does not occur.
These considerations particularly apply to vibration generators used for compaction of bituminous material in road construction as they are often subjected to temperatures of more than 100 degrees Centigrade.
Although the pressure source is separately mounted and thus spaced apart from the actual working cylinder, and hence works at normal temperatures, the operating temperatures in the working cylinder can reach an unbearably high level in the cases quoted above without any compensation taking place.
Thus German OS No. 2 607 190 covers a cooling system for a vibration generator in which the pressure fluid heated in the cylinder chambers is withdrawn and cool pressure fluid is fed to them.
However, a disadvantage is that the flushing out of the heated pressure fluid can only be dependent on pressure, i.e. the flushing quantity is always the same although with a constant pressure and small strokes less heat is generated than with long strokes. Thus with short strokes this design operates with uneconomically high flushing quantities. Metering of the necessary flushing quantity dependent on the stroke can only be achieved by relatively complicated methods since for this an additional external control mechanism with two sleeves is necessary.
For the periodic alternating pressurisation of the two cylinder chambers there are basically two further solutions available, one being the use of an electro-hydraulic control valve as for example covered by German OS No. 1 634 556 and also by the previous German Patent Application No. P 27 32 934.6 of the applicant, or also the SIREX "Impulse Generator" type of cylinder control, which converts a continuous supply of pressure fluid fed to it into one or two pulsating currents, so that the piston faces of cylinders connected to these currents can be pressurised on both sides.
In all cases however three basic troubles occur with the cylinder/piston drive unit which cause deviation of the oscillating piston in the cylinder after a short period of operation unless suitable counter measures are taken. These troubles are:
(a) internal and external leaks; PA1 (b) asymmetry of the mass relationships, the effect of gravity and that of external forces on all the components fixed to the parts oscillating in relation no each other, i.e. the piston and cylinder; and PA1 (c) with the above uses the external effort to be applied is only delivered in one direction, likewise causing asymmetry and thus a tendency to deviation by the oscillating piston.
Troubles (b) and (c) lead to an unsymmetrical pressure pattern on the cylinder piston faces. Since the flow quantity depends on pressure, both with electro-hydraulic servo valves with flow regulation characteristics and also with the SIREX Impulse Generator as mentioned above with a set opening cross-section, there result from the operating conditions of the servo valve or the impulse generator in conjunction with the internal and external leaks a rapid deviation of the piston from the precise oscillation centre and unsymmetrical oscillation amplitudes per cycle.
To avoid this inevitable deviation of the piston a centring action must be carried out. Up to now this has been done by detecting movement of the piston by a stroke detector and regulating by feedback and the comparison of desired and actual values to bring the piston back to the desired oscillation centre and amplitude.
Thus with the ramming device covered by German OS No. 1 634 556 with a linear generator an electrical stroke detector is used to determine the actual stroke, the value of which is compared with the desired value in the control circuit. Dependent on results of the comparison a correction operation is applied to the electro-hydraulic control valve to adjust the desired stroke of the machine.
Also with the rammed material driving process covered by the previous German OS No. 27 32 934.6 a signal emitter is used as the stroke detector to carry out the centring operation.
To sum up, it can be seen that with vibration generators hitherto used and of the type described the movement of the piston is monitored by means of an electrical, inductive, capacitive or potentiometer-type stroke detector and is controlled by feedback.
A disadvantage with the centring devices based on an adjustment system is that the components of the electrical adjusting circuit are relatively complicated and thus relatively costly, thus having a considerable effect on the price of such a vibration generator. Apart from this stroke detectors hitherto used are very liable to break downs so that particularly under heavy duty conditions, as for example occur in a machine used for stone breaking, they are frequently damaged and fail to work. In addition to this, at the place of use of such machines dust can have a serious effect on the proper operation of such a detector so that corresponding and expensive measures must be taken to seal it off.